1. Field of the Invention
The present invention relates to a method of manufacturing a flash memory device, and more particularly to, a method of manufacturing a flash memory device which can prevent a smiling phenomena of an ONO layer and a bird's beak phenomena of a tunnel oxide film.
2. Discussion of Related Art
In general, a flash memory device forms gates in a cell region and a peripheral circuit region and then performs a re-oxidation process for recovering an etched damage. After performing an ion implantation process for forming a cell transistor junction and a peripheral transistor junction, the flash memory device performs a source/drain annealing process for ion-activating. The re-oxidation process recovers an edge of a tunnel oxide film and a surface of a semiconductor substrate which have damaged due to a etching process for forming a gate. Here, the re-oxidation process is performed to improve a retention characteristic, one of intrinsic characteristics. Furthermore, an oxide film formed by the re-oxidation process acts as a barrier for relieving the damage on the semiconductor substrate up to a certain point during performing the source/drain ion implantation process as a succeeding process.
FIG. 1 is a cross-sectional diagram illustrating a unit cell transistor formed by a method of manufacturing a flash memory device of the conventional art.
Referring to FIG. 1, a gate of a cell transistor is formed on a cell region of a semiconductor substrate 11 by stacking a tunnel oxide film 12a, a first polysilicon layer 13, a lower oxide film 14-1, an intermediate nitride film 14-2, an upper oxide film 14-3, a second polysilicon layer 15, a tungsten silicide layer 16, and a cap insulation film 17, according to a gate forming process. Although it is not shown, gates of a high voltage transistor and a low voltage transistor are formed in the peripheral circuit region. In the cell region, the first polysilicon layer 13 acts as a floating gate, an ONO layer 14 comprised by stacking the lower oxide film 14-1, the intermediate nitride film 14-2, and the upper oxide film 14-3 acts as a dielectric film. The second polysilicon layer 15 and the tungsten silicide layer 16 act as a control gate, and the cap insulation film 17 acts to prevent the tungsten silicide layer 16 from being oxidated during the succeeding thermal process by being formed with an oxide pattern or a nitride pattern.
During the gate forming process, the re-oxidation process is performed to recover the part damaged by the etching process. In response to this, a sidewall oxide film 18 is formed on the gate sidewall and the semiconductor substrate 11. The cell transistor junction 19 is formed by light doped drain LDD ion implantation process. Although it isn't shown, a LDD region of a low voltage transistor is formed in the peripheral circuit region. After then, a source/drain junction of the peripheral circuit transistor is formed by performing the source/drain ion implantation process, and a source/drain annealing process is performed for ion-activating all the source/drain junction formed in the cell region and the peripheral circuit region.
In the aforementioned method of manufacturing the conventional flash memory device, the re-oxidation process and the source/drain annealing process have been performed by a furnace thermal oxidation method. According to this, the re-oxidation process takes approximately 6 hours and the source/drain annealing process takes approximately 4 hours. The processes performed in twice which take a long time cause a bird's beak at the edge part “A” of the tunnel oxide film 12a, and a smiling phenomena at the edge part “B” of the ONO layer 14.
Especially, the re-oxidation process is performed in the condition of having a plasma damage which Si-dangling bond is cut off from an etched surface of the first and second polysilicon layers 13, 15 by a gate etching process. As a result of this, the Si-dangling bond easily reacts upon oxygen and thus a sidewall oxidation of the first and second polysilicon layers 13, 15 is rapidly progressed. During this, the sidewall oxidation of the first and second polysilicon layers 13, 15 is more rapidly progressed near the ONO layer 14 and the tunnel oxide film 12a, which causes the bird's beak and the smiling phenomena thereby. Furthermore, the sidewall part “C” of the tungsten silicide layer 16 is oxidated. Accordingly, the last gate profile has a negative type after completing the re-oxidation process.
The bird's beak on the tunnel oxide film and the smiling on the ONO layer cause a charge leakage and decrease of coupling ratio important in the flash memory. As a result of those, the device becomes worse in the manner of an electric characteristic. Moreover, the oxidation of the tungsten silicide layer 16 increases a surface resistance of the control gate. Therefore, the flash memory device manufactured by the conventional method degrades a charge retention characteristic and reliability characteristic as well as programming and erasing characteristics. Furthermore, in the conventional method, it takes a long time to perform the re-oxidation process and the source/drain annealing process, which results to drop down productivity.